DEVONIAN AND OLD RED PERIOD. 135 
North America. It seems quite possible, also, that the so- 
called ‘‘ Carboniferous Slates” of Ireland correspond with 
this group, and that the former would be more properly re- 
garded as forming the summit of the Devonian than the 
base of the Carboniferous. 
In no country in the world, probably, is there a finer 
or more complete exposition of the strata intervening be- 
tween the Silurian and Carboniferous deposits than in the 
United States. The following are the main subdivisions 
of the Devonian rocks in the State of New York, where 
the series may be regarded as being typically developed 
fig. 67) :— 
: (1) Cauda-Galli Grit and Schoharie Grit.—Considering the 
“Oriskany Sandstone” as the summit of the Upper Silurian, 
the base of the Devonian is constituted by the arenaceous 
deposits known by the above names, which rest quite conform- 
ably upon the Silurian, and which represent the Lower 
Devonian of Devonshire. The Cauda-Galli Grit is so called 
from the abundance of a peculiar spiral fossil (Spzrophyton 
cauda-Galli), which is of common occurrence in the Carbon- 
iferous rocks of Britain, and is supposed to be the remains 
of a sea-weed. 
(2) The Corniferous or Upper Helderberg Limestone. — A 
series of limestones usually charged with considerabie quan- 
tities of siliceous matter in the shape of hornstone or chert 
(Lat. cornu, horn). ‘The thickness of this group rarely exceeds 
300 feet; but it is replete with fossils, more especially with 
the remains of corals. The Corniferous Limestone is the 
equivalent of the coral-bearing limestones of the Middle De- 
vonian of Devonshire and the great ‘ Eifel Limestone” of 
Germany. 
(3) The Hamilton Group—consisting of shales at the base 
(‘‘ Marcellus shales”); flags, shales, and impure limestones 
(‘Hamilton beds”) in the middle; and again a series of shales 
(‘Genesee Slates”) at the top. The thickness of this group 
varies from 200 to 1200 feet, and it is richly charged with 
marine fossils. 
(4) The Portage Group.—A great series of shales, flags, and 
shaly sandstones, with few fossils. 
(5) The Chemung Group—Another great series of sand- 
stones and shales, but with many fossils. The Portage and 
Chemung groups may be regarded as corresponding with the 
Upper Devonian of Devonshire. The Chemung beds are 
succeeded by a great series of red sandstones and shales—the 
